Immune tolerance to self-antigens is the key issue in many important clinical situations, such as cancers. Most tumor antigens are self-antigens de novo expressed in tumor cells, the tolerance that develops to such "neo self-antigens" permit tumor growth. Poor appreciation for the subtleties of tolerance induction is almost certainly a major reason why cancer immunotherapy has met with limited success. Although the timing of first antigen contact is a critical determinant for generating graft tolerance or oral tolerance, much remains to be understood regarding tolerance to self-antigens expressed during different phases of life. We have developed a transgenic model in which a single-copy surrogate autoantigen (Tac) is conditionally expressed in the thyroid. The well defined CD8 T cell, CD4 T cell and B cell epitopes are placed into the surrogate autoantigen for assessing specific immune responses. Our preliminary studies suggest that embryonic antigen expression results in strong CD8 T cell tolerance while adult antigen delays the CD8 T cell tolerance. In this application, we will continue to explore this model by assessing CD4 T cell and B cell tolerance to conditionally expressed Tac transgene. Based on the preliminary data, we hypothesize that different mechanisms may be involved in the induction of T cell tolerance at different developmental stages, e.g. central mechanisms for embryonic antigen and peripheral mechanisms for adult antigen. Using a variety of techniques, we will investigate the tolerance mechanisms that play a role under various conditions. The inducible transgenic model can also be used to represent the appearance of a tumor-associated antigen (TAA) or tumor specific antigen (TSA) before or at the time of neoplastic transformation. In tumor challenge models with Tac+ tumor cells, the Tac antigen will serve as a TAA in mice with previous Tac expression but as a TSA in mice without previous Tac expression. The anti-tumor activity will be assessed by tumor growth and specific immune responses will be evaluated in each group of mice. We predict that previous antigen expression impairs anti-tumor activity to different extents, depending upon the timing of initial antigen expression. TSA has been considered as a better target for cancer immunotherapy, but we hypothesize that long-term TSA expression may result in tolerance to TSA. A better understanding of tolerance induction to self-antigens expressed in normal and tumor cells may lead to key innovations in cancer immunotherapy. PUBLIC HEALTH RELEVANCE: For over a century researchers and clinicians have attempted to use the immune system to attack cancers. On rare occasions results have been stunningly successful but most of the time they have been disappointing. A large part of the problem is that tumors are part of "self" and are therefore subject to mechanisms that induce tolerance to the tumor cells. A better understanding of tolerance would point to innovative ways of breaking tolerance to tumor cells, leading of attack of cancer cells. This application focuses upon the induction of tolerance as it directly relates to immune responses to cancer.